The present invention relates to inspection systems. More particularly, the present invention relates to improved optical inspection systems for inspecting flat-panel substrates or displays, such as liquid crystal displays (LCDs).
Relatively simplistic methods have previously been proposed for inspection of LCD panels. One such method is described in U.S. Pat. No. 5,650,844. The '844 patent describes using a single CCD camera having 1534×1024 pixels to capture an image of an entire LCD panel having 640×480 pixels. The technique described simply discloses determining whether LCD pixels are on or off.
There are many real-world drawbacks to the methods described in the '844 patent. One such drawback is that LCD panels have different illumination properties at different viewing angles. Accordingly, when attempting to capture an entire LCD panel with a single camera, as taught, the image acquired by the camera will have viewing angles artifacts at different parts of the LCD panel that would interfere with inspection of the LCD panel. Other optical drawbacks include geometric distortion of the image of the LCD panel.
Another drawback is that the '844 patent appears to assume that the panel and camera are perfectly aligned, that is that there is no misalignment angle between them. In practice, in a typical manufacturing line, the misalignment angle between the panel and camera is not insignificant. For example, typical assembly line positioning systems would be accurate to <5 degrees, <10 degrees or the like. It would be prohibitively expensive and very time consuming (in the manufacturing line) to have a positioning system that could provide a misalignment accuracy, for example <0.01 degrees, small enough so that the misalignment angle could be disregarded. Because typical manufacturing lines do not have such accurate positioning systems, the '844 does not provide a practical solution.
Yet another drawback is that the '844 patent does not address detection of defective sub-pixels of an LCD pixel, and does not address the detection of defects in portions of sub-pixels of an LCD panel. The '844 patent only appears to describe inspecting contrast ratios in LCD pixels. In contrast, in the industry, manufacturers are concerned with inspecting sup-pixels of a display pixel, for example red, blue, and green sub-pixels. Further, manufacturers are interested in identifying sub-pixels which are partially defective so they can repair the defects.
Simply using higher resolution camera along with the teaching of the '844 patent, would still have drawbacks. One such drawback is the viewing angle artifacts described above. Another drawback is that there is no teaching about processing images of different colors differently. In contrast, the inventors of the present invention have discovered that when inspecting test scenes (images) while exciting only red, blue, or green pixels, it is highly desirable to use different calibration parameters, driving parameters, and thresholds to provide accurate defect position identification.
Simply using one camera to take multiple images of a display panel or multiple cameras to take one image of a display panel along with the teaching of the '844 patent, would still have drawbacks. One drawback is that in practice, it is virtually impossible to perfectly align the display pixels of each camera to a display panel. Accordingly, processing of such images using the teaching of the '844 patent would be inaccurate as each camera image will have a different misalignment angle. Further, using a single camera multiple times would be a very time consuming process, and unacceptable for use on a manufacturing line.
In light of the above, what is desired are methods and apparatus for detecting defects in sub-pixels of a display panel, without the drawbacks described above.